1. Field of the Invention
The present invention relates to a pulse modulation scheme which repeatedly generates optical pulses of a predetermined period as seed light for obtaining high-power output light, and an optical fiber laser to which such a scheme is applied.
2. Related Background of the Invention
Attention has currently been paid to processing techniques using laser light, so that demands for high-power laser light sources have been increasing in the fields of processing, medical care, and the like. Among various laser light sources, those attracting attention in particular are optical fiber lasers. Such an optical fiber laser employs an amplification optical fiber, whose core is doped with a rare-earth element such as Yb (ytterbium), Er (erbium), or Tm (thulium), as an optical amplification medium. When pumping light is fed into this amplification optical fiber, seed light propagating therethrough is amplified. This allows the amplification optical fiber to emit laser light by outputting high-power amplified light or causing laser oscillation with a resonator structure. The optical fiber lasers are advantageous in that the laser light is confined within the optical fiber and thus is easy to handle, that their thermal dissipation is so good that no large-scale cooling equipment is necessary, and so forth.
As described above, the optical fiber lasers employ fibers doped with rare-earth elements. Among such rare-earth-element-doped fibers, Yb-doped fibers yield a high conversion efficiency and thus have widely been utilized as high-power outputting amplification optical fibers. Yb is pumped with pumping light as with the other rare-earth elements. On the other hand, the part of pumping light left unabsorbed within an amplification optical fiber is emitted from the other end of the amplification optical fiber.
When a resonator structure utilizing a fiber Bragg grating (FBG), a mirror reflector, or the like at each end is employed as a structure of an optical fiber laser, for example, pulse modulation is performed by arranging an optical switch or acousto-optic modulator (AOM) within the resonator. On the other hand, a MOPA (Master Oscillator Power Amplifier) type optical fiber laser such as one disclosed in Japanese Patent Application Laid-Open No. 2007-042981 directly or externally modulates a seed light source which outputs light to be amplified, so as to perform pulse modulation, and amplifies the resulting optical pulses, thereby yielding high-power output light. In either structure, the output obtained by pulsing the seed light is much higher than that in the case of a continuous-wave operation (CW operation), whereby nonlinear phenomena such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) may emerge.